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Tach is at 2500 to 3000 at 55 60 MPH ??

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Old 10-12-2010, 07:58 PM   #1
78 C3 Light Corvette Blue
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Default Tach is at 2500 to 3000 at 55 60 MPH ??

Is this normal ??? 78 L82 Automatic
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Old 10-12-2010, 08:22 PM   #2
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It can be if You have gears around 3.55:1 in the rear.
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Old 10-12-2010, 08:51 PM   #3
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There is no tag to tell me the gears. I think back then the rpm's ran a bit high. I remeber a 69 Firebird being high. I don't think as high as this Vette.
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Old 10-12-2010, 09:01 PM   #4
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My 78 with 3.08 gears turns 2,400 to 2,500 at 60
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Old 10-12-2010, 10:53 PM   #5
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whats your tire height?
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Old 10-12-2010, 11:23 PM   #6
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255\60\15
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Old 10-12-2010, 11:42 PM   #7
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Quote:
Originally Posted by 78 C3 Light Corvette Blue View Post
255\60\15
Well assuming you have a th400 with a 3rd gear of 1.1 and 27" tire.


so you must have a 3.55 to a 4.11
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Old 10-13-2010, 12:03 AM   #8
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Many 78s had 3:70 gears.
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Old 10-13-2010, 12:31 AM   #9
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my 77s tire height is 27.4 and i was at 3000 rpm at 61 mph. and calculated it out to be 3.91 gears in mine so yours are going to be close to mine
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Old 10-13-2010, 06:15 AM   #10
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my 70 with 3:70 gears tachs at 2500 @ 55 mph in high gear.. stock 27 inch tires..
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Old 10-13-2010, 07:08 AM   #11
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With stock tire height (255/60/15 or 225/70/15) my L-82 4 speed (all stock) and 3.70 gears should turn exactly 2,860 RPM's at 60 mph which it does. At 70 MPH, my car is turning approximately 3,500 RPM. Now you know the reason so many 4 speed owners convert to 5 speeds with the overdrive. It is normal! One of these days I will do the conversion as well. Hope that helps!
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Old 10-13-2010, 07:36 AM   #12
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Quote:
Originally Posted by wombvette View Post
Many 78s had 3:70 gears.
sounds like 3:55 or 3:70 to me..check the numbers on the underside of the differential housing, there should be the code there...

Last edited by 73jst4fun; 10-13-2010 at 07:45 AM.
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Old 10-13-2010, 07:41 AM   #13
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Quote:
Originally Posted by meinthe73 View Post
Well assuming you have a th400 with a 3rd gear of 1.1 and 27" tire.


so you must have a 3.55 to a 4.11
Were did you find that calculator? That is pretty slick!
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Old 10-13-2010, 08:23 AM   #14
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Quote:
Originally Posted by Timsride View Post
It can be if You have gears around 3.55:1 in the rear.
Standard axle ratio for a 78 L82 Auto was 3.55.

L48 standard was 3.36 for 4spd and 3.08 for auto

4spd L82's had the 3.70 as standard.

so it sounds about right.

Last edited by karol; 10-13-2010 at 08:31 AM.
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Old 10-13-2010, 09:38 AM   #15
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Quote:
Originally Posted by 604 View Post
Were did you find that calculator? That is pretty slick!
just google rpm calculator. i think its the first one.
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Old 10-13-2010, 09:54 AM   #16
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Default Is it possible...

I recently purchased a 1977. 4 speed manual.

5200 rpm at 65 mph. Tires are 25.7 ".

3500 rpm at 45 mph.

Note: the actual rpm tested with a meter is roughly half of what the gauge shows.

So, what is causing the rpm gauge to read so high?

Last edited by 81_vette!; 10-13-2010 at 10:14 AM.
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Old 10-13-2010, 10:41 AM   #17
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Those are some little tires for a c3
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Old 10-13-2010, 10:55 AM   #18
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Quote:
Originally Posted by meinthe73 View Post
Those are some little tires for a c3
The car is lowered - so the height of the tires needed to be appropriate.

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Old 10-13-2010, 11:06 AM   #19
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Quote:
Originally Posted by 81_vette! View Post
The car is lowered - so the height of the tires needed to be appropriate.

Nice... I said that was a small tire because that will increase rpm. Looks good, but just my thoughts get a little bigger tire for the rear, but still looks good.
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Old 10-13-2010, 07:05 PM   #20
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Thanks Guys
My mechanic was wondering what kind of gears were in there? I thought it was normal. How ever I thought I would ask. Get some feed back with you that own and drive these cars.
Thanks again
I will look for the code. I have to replace a clip on one of the yokes.
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